The invention relates to controlling electrical switchgear. More particularly, the invention relates to continuously and automatically optimizing switchgear performance.
In a power distribution system, switchgear are typically employed to protect the system against abnormal conditions, such as power line fault conditions or irregular loading conditions. There are different types of switchgear for different applications. A fault interrupter is one type of switchgear. Fault interrupters are employed to automatically open a power line upon the detection of a fault condition.
Reclosers are another type of switchgear. In response to a fault condition, a recloser, unlike a fault interrupter, rapidly trips open and then recloses the power line a number of times in accordance with a set of time-current curves. Then, after a predetermined number of trip/reclose operations, the recloser will xe2x80x9clock-outxe2x80x9d the power line if the fault condition has not been cleared.
A breaker is a third type of switchgear. Breakers are similar to reclosers. However, they are generally capable of performing only a single open-close-open sequence, and the currents at which they interrupt current flow are significantly higher than those of reclosers.
A capacitor switch is a fourth type of switchgear. Capacitor switches are used for energizing and de-energizing capacitor banks. Capacitor banks are used for regulating the line current feeding a large load (e.g., an industrial load) when the load causes the line current to lag behind the line voltage. Upon activation, a capacitor bank pushes the line current back into phase with the line voltage, thereby boosting the power factor (i.e., the amount of power being delivered to the load). Capacitor switches generally perform one open operation or one close operation at a time.
As switchgear contacts come into proximity with one another (i.e., during a closing operation) or when the contacts first separate (i.e., during an opening operation), some amount of arcing occurs between the contacts. Arcing can cause an excessive amount of heat to build up on the surface of the contacts, which can cause the contacts to wear-out at an excessively fast rate. Arcing can also strain or damage system components such as power transformers. Therefore, arcing is highly undesirable.
In general, all switchgear, irrespective of type, attempt to minimize arcing. Some switchgear designs attempt to accomplish this by driving the switchgear contacts apart (i.e., during an opening operation) or together (i.e., during a closing operation) as fast as possible. The theory behind this approach is that if the amount of time the contacts spend in close proximity to one another is minimized, arcing is also minimized. In practice, this strategy is flawed, particularly during closing operations, because the contacts tend to bounce when they come into physical contact with each other, with the amount of bounce increasing as the relative velocity of the contacts increases. Contact bounce, in turn, leads to the generation of undesirable transient voltage and current events.
A more effective method for minimizing arcing and minimizing the generation of transients is to synchronize the initiation of the switchgear operation so that the actual closing or opening of the contacts occurs when the AC voltage or current across the contacts is at zero volts or zero amperes, respectively. For example, in FIG. 1, it is preferable that a closing of the contacts occurs when the AC voltage waveform 100 passes through a zero-voltage crossover point, such as point A. Generally, for true synchronous operations, it is preferable to close at a voltage zero across the switchgear contacts and to open at a current zero to minimize arc time. Normal arc interruptions occur at a current zero. For a capacitor switch application, the capacitor load current leads the voltage by 90 electrical degrees. Therefore, the current waveform does not need to be monitored and it can be assumed that at a voltage zero the current is at a peak and at a current zero the voltage is at a peak. For true synchronous operations for other applications, both the voltage waveform and current waveform need to be monitored to achieve the proper synchronous timings.
Present switchgear designs that employ a synchronizing method generally do so by predefining a fixed amount of time t1, where t1 is equal to a presumed AC voltage waveform period T less an amount of time t2 corresponding to an approximate amount of time required to complete the switchgear operation. This is referred to as fixed time synchronization. For example, in FIG. 1, if the AC voltage waveform is operating at 60 Hz, the period T of the AC waveform 100 is 16.66 msec. If the predefined time t2 is 11.66 msecs, then t1 is 5 msecs. Accordingly, if a switchgear employing this method receives a command to initiate a close operation, the switchgear will detect a next zero-voltage crossover point, such as crossover point B in FIG. 1, then wait t1 msecs, which corresponds with point C in FIG. 1, to initiate the switching operation. Likewise, if an open command is received, the switchgear will detect a next zero current crossover point and determine an appropriate opening point that is somewhat similar to the timing sequence described above for the closing operation. The opening point is determined such that a contact opening gap sufficient to interrupt the flow of current and withstand the power system recovery voltage to prevent reignitions or restrikes is established at the next zero current crossover. From here on, the discussion will focus on synchronized voltage switching. However, it will be understood by one skilled in the art that switching could also be synchronized with the current waveform on opening.
Unfortunately, the fixed time synchronization method does not always produce accurate results. First, the AC voltage waveform 100 rarely propagates at exactly 60 Hz. In fact, it generally fluctuates slightly above and below 60 Hz. Accordingly, the period T of the AC voltage waveform 100 will fluctuate. Therefore, initiating a switching operation at point C does not always guarantee a synchronized opening or closing operation (i.e., an operation that is synchronized with a zero-voltage crossover point). Second, conditions such as ambient temperature can affect the dynamic friction of the mechanism and change the actual amount of time that it takes for the contacts to complete the switching operation. Therefore, the amount of time represented by t2 may fluctuate with temperature. Thus, once again, initiating the switching operation at point C is not likely to consistently result in a synchronized opening or closing operation. Third, over the life of the switchgear, the distance the contacts must travel during a switching operation generally increases. This is due to ordinary contact wear and wear from the components of the mechanism. As the contact travel distance increases, it becomes less likely that initiating the switching operation at point C as a function t1, t2 and T will result in a synchronized switching operation. Therefore, present switchgear designs that employ the fixed time synchronization method must be manually recalibrated frequently to maintain their precise synchronous timing.
In the particular case of a capacitor switch, minimizing arcing and minimizing the generation of transients is especially important. That is because even small inaccuracies in synchronizing a switching operation with a zero-voltage crossover point on the AC voltage waveform can result in arcing and/or transients that involve thousands of amperes and volts. Therefore, an enormous demand exists for a switchgear design, particularly a capacitor switch design, that provides automatic compensation for more accurate, point-on-wave switching operation control, to better assure zero-voltage switching operations to minimize transient effects.
A system employing the present invention provides precise, point-on-wave switching performance by employing a closed-loop feedback, microprocessor-based motion control design. By employing a closed-loop feedback, microprocessor-based design, the system can monitor and optimize switchgear contact motion (i.e., position and velocity) during a switching operation, thereby assuring a more accurate switching operation. Moreover, the closed-loop feedback design intrinsically self-compensates for the effects of factors such as ambient temperature, AC waveform fluctuations, and changes in the physical condition of the switchgear. In addition, the system can optimize various motion control parameters both during and subsequent to a switching operation, to better assure that present and future operations are more accurately synchronized with the AC voltage or current waveform of the AC electrical circuit.
The system promises to minimize arcing and transients during switching operations, and to provide accurate, consistent point-on-wave switching. The system may continuously monitor and optimize, in real-time, the moving components of the system, based on present switching operation performance, to assure more consistent and accurate, point-on-wave switching.
The system also may periodically optimize the moving components based on past switching operation performance, to assure more accurate, point-on-wave switching operations.
In accordance with one general aspect of the invention, a closed-loop feedback control system for electrical switchgear that moves one contact relative to another contact to switch power on and off in the AC electrical circuit includes a position sensor and a processor. The position sensor is operatively coupled to at least one of the two contacts to produce contact position information. The processor, in turn, is configured to receive and analyze the contact position information to control contact motion to provide AC waveform synchronized switching.
Embodiments may include one or more of the following features.
The processor may control a single AC phase of the AC electrical circuit. Likewise, the AC electrical circuit may include a poly-phase circuit and the processor may control each phase of the AC electrical circuit. The AC electrical circuit may include a power line.
The processor may control contact motion based on a comparison between the contact position information and a target contact position. The target contact position may be based on prior contact position information.
The processor may use the contact position information to determine erosion in electrical switchgear components or residual contact life.
The closed loop feedback control system may include a hermetically-sealed bottle that houses the switchgear contacts. The processor may use the contact position information to detect fractures or leaks in the bottle.
The feedback system may be part of a capacitor switch. The capacitor switch may include a latching device that maintains the contacts in one of an open stable position in which electrical current does not flow through the contacts or a closed stable position in which electrical current flows through the contacts.
The capacitor switch may include a mechanical trip mechanism that allows an operator of the capacitor switch to manually open switch contacts. The mechanical trip mechanism, when activated by the operator, may open switch contacts at least as fast as the closed loop feedback control system.
The mechanical trip mechanism may include a trip lever, a handle, a compression spring, a trip plunger, a spring plate, and a trip finger. The handle, when pulled by the operator, may rotate the trip lever. The trip plunger may couple the trip lever to the compression spring such that rotation of the trip lever pushes the trip plunger in a direction that compresses the compression spring. The spring plate may couple the compression spring to the movable contact. The trip finger may rotate away from the compression spring when contacted by the trip plunger to release the spring plate and move the movable contact away from the other contact.
The mechanical trip mechanism may also include a return spring that, after operator activation, may automatically reset the mechanical trip mechanism independently from closed loop feedback control system operations. The mechanical trip mechanism may be reset by the operator after operator-activation. Furthermore, the contacts may remain open until the closed loop feedback control system moves the contacts closed.
In accordance with yet another general aspect of the invention, a latching device used in an electrical switchgear includes a shaft operable to move along a shaft axis, a piston operable to move along a piston axis, a biasing device, and a linkage. The shaft is coupled to a contact of the switchgear and operable to move along the shaft axis between a first stable position in which an electrical path including the contact is closed and a second stable position in which an electrical path including the contact is open. The biasing device is coupled to the piston to exert a biasing force on the piston along the piston axis and the piston, in turn, is coupled to the shaft through the linkage. The linkage is configured such that the biasing force on the piston is transferred to the shaft to bias the shaft to one of the stable positions.
Embodiments may include one or more of the following features.
The shaft may be operable to move along the shaft axis between the first stable position, the second stable position, and a third stable position in which an electrical path including the contact is open. Furthermore, the piston axis may be perpendicular to the shaft axis.
The latching device may further include a biasing adjustment that adjusts the biasing force of the biasing device. Likewise, the latching device may include a biasing retainer that fixes the biasing force of the biasing device.
The latching device may include a second piston operable to move along a second piston axis, a second biasing device, and a second linkage. The second biasing device is coupled to the second piston to exert a second biasing force on the second piston along the second piston axis and, in turn, the second piston is coupled to the shaft through the second linkage. The second linkage is configured such that the second biasing force is transferred to the shaft to bias the shaft to one of the stable positions. The shaft may be operable to move along the shaft axis between the first stable position, the second stable position, and a third stable position in which an electrical path including the contact is open.
The biasing device may include a spring. Furthermore, the shaft may be insulated from the contact.
The first stable position may be constrained such that the biasing force is maximally coupled to the contact through the shaft. The constraint may ensure that the electrical path is closed in the first stable position. The constraint may account for contact erosion. Likewise, the second stable position may be constrained such that the biasing force is maximally coupled to the shaft along the shaft axis. The piston may be operable to move a distance that ensures that the electrical path is closed in the first stable position and that the electrical path is open in the second stable position.
The latching device may further include a shock absorbing system that includes at least one shock absorbing piston operable to move along a shock absorbing axis and at least one shock absorbing biasing device. The shock absorbing piston couples to the shaft and the shock absorbing biasing device is coupled to the shock absorbing piston to exert a shock absorbing biasing force on the shock absorbing piston along the shock absorbing axis. The shock absorbing piston is configured such that the shock absorbing biasing force dampens contact bounce at at least one stable position. The shock absorbing axis may be parallel to the shaft axis. Furthermore, the shock absorbing biasing force may prevent contact bounce at at least one stable position.
The shaft may be coupled to multiple contacts of the switchgear. Each contact may correspond to a phase of polyphase AC power.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.